Optical brighteners are added to certain photographic elements, for example graphic arts black and white photographic paper, to produce a white appearing background. Photographically useful optical brighteners can be oil-soluble or water-soluble compounds. Oil-soluble optical brighteners can be utilized in aqueous photographic compositions as dispersions in oil or in a polymer latex, see for example, U.S. Pat. No. 4,230,716 to Chen. But such oil-soluble brighteners are very expensive and are not usually used as they substantially increase the cost of the product. Further, oil-soluble optical brighteners tend to self quench (i.e. lose some of their fluorescence) unless the oil phase of the dispersion is very dilute, which is generally undesirable as it increases the solvent load in the photographic element. Also, oil dispersed optical brighteners tend to retain sensitizing dyes which can cause stain in the photographic element.
Water-soluble optical brighteners are a constituent of common laundry detergent and consequently are available at relatively low cost. Because of their water-solubility, such compounds can be added directly into an aqueous photographic coating composition and do not need to be dispersed as an oil phase, thereby providing additional cost savings in the manufacture of the photographic element. However, such water-soluble optical brighteners tend to diffuse out of the photographic element or from one layer of the photographic element to another during coating and processing of the element, thereby reducing the effectiveness and specificity of the brightener. For this reason, most photographically useful optical brighteners, are ballasted. Most ballasted optical brighteners are expensive compared to the unballasted versions.
Usually, some of the water-soluble optical brightener is washed out during processing of the film and the retained optical brightener performs the required brightening. With low cost water-soluble brighteners, even with the wash loss, photographic elements containing the optical brightener are reasonably priced. However, the leached out brightener continuously seasons the developer solution, requiring it to be replenished more frequently to ensure the uniformity of the processed product.
Therefore, there exists a need to provide inexpensive, water-soluble optical brighteners for photographic coatings which do not diffuse from one layer to another nor leach into the processing solution.
Two approaches reported in the art to resolve this problem are described in U.S. Pat. Nos. 3,677,762 to Amano et al and 3,749,707 to Hove et al. In U.S. Pat. No. 3,677,762, a high molecular weight polymeric optical brightener is described. The cost of manufacturing such specialized functional polymers is generally even more expensive than the use of oil-soluble optical brighteners. Also, polymeric optical brighteners tend to undergo self quenching of fluorescence as the optical brightener moieties come close together in an oily polymer composition. U.S. Pat. No. 4,943,519 describes compositions of such polymeric optical brighteners or latex polymeric optical brighteners. In U.S. Pat. No. 3,749,707, the optical brightener is reacted with gelatin to reduce the water solubility of the compound. In example 3 of the '707 patent, the optical brightener, 2,2'-disulfo-4,4'(2,4-dichloro-s-triazine-6-yl-amino)-stilbene, is reacted with gelatin. It is reported that the gelatin derivative prepared became water insoluble after storing. This is due to crosslinking of the gelatin and can limit the shelf life and utility of the brightener/gelatin combination.